1. Field of Invention
The present invention relates to signal processing; and more particularly; to a system and method for correcting imbalances between In-phase (I) and Quadrature (Q) phase components of radiant energy signals.
While the invention is subject to a wide range of applications, it is especially suited for use in balancing (I) and (Q) components of radar receiver return signal, and will be particularly described in that connection.
2. Description of Related Art
In practically all moving target indication (MTI) radar systems the incoming signals are divided into I and Q components to detect the return signal vector phase shift caused by target motion. The I and Q components of the detected signals are processed and converted to complex numbers, with the I components corresponding to real numbers, and the Q components corresponding to imaginary numbers. The received components in the form of an IF signals are down-converted to base band by one or more converters the last one being an I/Q converter. The output of the I/Q converter is low pass filtered to remove the sum product and retain the difference frequency terms: EQU I(t)=A cos .omega.t EQU Q(t)=A sin .omega.t
where t=time in seconds, A=amplitude, and .omega.=radians per second,
This pair of signals can be treated as a complex signal: EQU I(t)+JQ(t)=A exp (J.omega.t)
where J is the square root of minus one.
This ideal signal is valid only if the gains of the I and Q paths are equal; and if the phase difference between the I and Q channels is 90 degrees. Gain or phase imbalances of any kind will cause an image response at the negative of the signal frequency. However, because of manufacturing tolerances, ambient conditions, and aging factors, for example, the particular devices that are involved in converting the return signals to the Q components, at times, generate Q components that do not correspond to the amplitude of the I component, or generates Q components that are less than or greater than ninety degrees out-of-phase with the I components. Image power of radar receiver is typically 25-35 dB below the signal power for typical quadrature converter chains with amplitude imbalances of 0.5 dB, and phase imbalances of three degrees.
Correction of such in,balances may be accomplished by adjusting the phase and amplitude of the analog signal either in the signal path and/or the local oscillator path prior to quadrature mixing using feedback; or correcting in a feed forward manner the phase and amplitude of the complex signal after conversion. Correction after conversion may be either analog or digital. Since analog phase correction is not convenient at frequencies near DC, the known correction schemes are subsequent to the A/D conversion of the components (i.e., digital). A side benefit of I/Q correction is DC offset removal, which is obtained readily from the techniques used to measure the phase and amplitude imbalances.
One well known and effective imbalance correction technique is described in an article by F. E. Churchill et al. published in Volume AES-17, No. 1, dated January 1981 of IEEE Transaction of Aerospace and Electronic Systems. In this technique coefficients are derived from a test tone or pilot signal, which are applied to the complex digitized data. Measurement of the pilot tone to find the coefficients can be done in either the time or frequency domains. Prior to the present invention, the time domain process was restricted to four samples of the pilot tone spaced 90 degrees apart. This prior art technique requires a precise frequency relationship between the pilot tone and the pilot tone sampling rate. It is possible to overcome this disadvantage by performing calculations via FFT processing to remove the sample rate restriction. However, this complicates the radar receiver, particularly microprocessor based radar receiver controller mechanizations.
To overcome the above disadvantages, in excess of four times the data samples were used to get a better signal to noise ratio, and phase samples were taken at other than ninety degree spacing. However, a precise number of cycles of the phase samples of the test tone were still required to be sampled, restricting the test tone frequency/sampling rate to a specific number of cycles of the sampled test tone. Additionally, in calculating the correction coefficients, transcendental functions (that is sine and cosine functions) are required, which also may be burdensome for microprocessor mechanizations.
In light of the foregoing, there is need for a system and method for correcting imbalances in the I and Q components of a target return signal that operates as effectively as the known systems and methods, but is simpler, more efficient and can be implemented readily in microprocessor mechanizations.